Estrogen is a critical hormone in the human body because it regulates the growth, development and homeostasis of numerous tissues, including the regulation of mammalian reproduction and breast function, the central nervous and immune systems, skeletal physiology and vascular function. Our long-term goal is to elucidate the role of a novel intracellular, 7-transmembrane spanning, G protein-coupled estrogen receptor (GPR30) that we propose functions alongside the traditional estrogen receptor (ER) to regulate physiological responsiveness to estrogen. The specific hypothesis is that signaling through GPR30 regulates uterine function and cancer development. We base this hypothesis on our recent observations that 1) GPR30 represents a functional, estrogen-binding G protein-coupled receptor (GPCR), 2) GPR30 activates novel nuclear phosphatidylinositol signaling pathways and 3) GPR30 is specifically expressed in uterine glandular epithelium and overexpressed in endometrial cancer. The specific aims are: 1. Characterize estrogen-mediated cellular activation by GPR30. We will compare cellular signaling initiated by GPR30 and traditional ERs, stimulated by estrogen as well as estrogen analogs, and determine the effects of the clinically used kinase inhibitors Iressa and Lapatinib. 2. Animal models of GPR30 function in uterine biology and neoplasia. We will determine the developmental regulation of GPR30 in the normal and neoplastic mouse uterus. Using GPR30 knockout mice, we will determine the role of GPR30 in estrogen-dependent signaling in the uterus and how this regulates endometrial tumor development. 3. Evaluation of GPR30 expression in human endometrial cancer. We will investigate the role of GPR30 as a novel biomarker predictive of grade, stage and adverse outcome in intermediate and high-risk endometrial cancer. Characterizing the functions of this novel estrogen receptor in conjunction with translational clinical trials will contribute to our understanding of estrogen-induced growth and proliferation in the neoplastic uterus, leading to the development of GPR30 as a novel biomarker and as a target for diagnostic and therapeutic development.